A linear spool valve is a device commonly employed in work exchangers to obtain energy from one fluid stream and transfer that energy to another stream. It functions as a pump driven by fluid flow, and it normally comprises a body with a cylindrical cavity with an inner surface and piston adapted to reciprocate within the cavity in contact with the inner surface. Most known pistons are made of solid polymer material (not metal) in order to avoid intensive wear and tear of the valve body, which is normally made of steel.
Linear spool valves are often used in work exchangers for recovery of energy in reverse osmosis processes such as desalination. One example of such spool valve is disclosed in the Applicant's WO04111509. The use in reverse osmosis desalination imposes certain limitations on the valve's design because such processes often take place under high pressure differential between two ends of the piston and high temperature range. In particular, in course of a year, the seawater temperature is changed as a rule in a range from 10 to 35° C. The pressure across the piston is usually around 65-70 bars on one end of the piston and 1-2 bars on the other end of the piston. In order to withstand such pressure and temperature conditions, a very small clearance is allowed between the piston and the inner surface of the valve, which is preferably not greater than about 0.06 mm.